I watched a few guys attempting this shortcut and falling down repeatedly, anyway do it the RIGHT way like i did if you can its easyer then it looks after a few tries i just did all the jumps by reflex and bingo done, after you get the achiev and the chest you can cross to the other island and theres a champion guarding another small chest

I was so lonely up there near the chest , there was no 1 in sight, eventually got bored and dropped down.